Defining Selection


Given a table of values, such as C.Wunsch's data from GEOSECS Station 3, with an added column of a fictional date_time added to enable the examples: 
	date_time       press    temp     sal      O2  Sigma0
	------------------------------------------------------
	199408031130    5.000  18.334  33.570   5.970  24.096
	199408031145   25.000  12.848  34.159   6.990  25.773
	199408031200   49.000  11.070  34.523   6.060  26.394
	199408031230   99.000  11.093  35.090   5.340  26.831
	199408031330  149.000  11.906  35.487   5.020  26.990
       	 ...
A selection can be defined as choosing record instances which satisfy certain criteria. In other words, a selection determines which rows to include in operations which follow.

Example of a numeric selection:
Your selection is: press>50
which translates to "all values with a pressure value exceeding 50"

The result is: 
	date_time       press    temp     sal      O2  Sigma0
	------------------------------------------------------
	199408031230   99.000  11.093  35.090   5.340  26.831     correct, numerically
	199408031330  149.000  11.906  35.487   5.020  26.990      the values for press
        ...                                                           exceed 50

Example of a string selection:
Your selection is: date_time ge 199408031200
which translates to: "all values which occured at noon or after on August 3, 1994"
Note: string selections are accomplished by comparing values as they occur in the ASCII table

The result is: 
	date_time       press    temp     sal      O2  Sigma0
	------------------------------------------------------
	199408031200   49.000  11.070  34.523   6.060  26.394
	199408031230   99.000  11.093  35.090   5.340  26.831
	199408031330  149.000  11.906  35.487   5.020  26.990
       	 ...

Example of a how NOT to use a string selection:
Your selection is: press lt 50
which translates to "left-justifying, return all values with a pressure which would occur before 50 in the ASCII table"

The result is: 
	date_time       press    temp     sal      O2  Sigma0
	------------------------------------------------------
	199408031130    5.000  18.334  33.570   5.970  24.096      
	199408031145   25.000  12.848  34.159   6.990  25.773
	199408031200   49.000  11.070  34.523   6.060  26.394
	199408031330  149.000  11.906  35.487   5.020  26.990  correct!  "1" is lower than "5"
       	 ...
*** Lesson: When dealing with values which are numbers and which are meant to be used arithmetically, use a numeric selection.

Defining Projection

A projection can be defined as choosing which parameters to use from among all available parameters. In other words, you can determine to see only some of the columns in a table.

Example:
Your projection is: press, temp, O2
The result is: 
	  press    temp      O2
        -----------------------	
	  5.000  18.334   5.970
	 25.000  12.848   6.990
	 49.000  11.070   6.060
	 99.000  11.093   5.340
	149.000  11.906   5.020
        ...
These two query types can be used in combination to further specify a subset.

Example combining selections and projections:
Your subset is: temp < 15, press, temp, O2
The result is: 
	  press    temp      O2
        -----------------------	
	 25.000  12.848   6.990
	 49.000  11.070   6.060
	 99.000  11.093   5.340
	149.000  11.906   5.020
        ...
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